The invention relates generally to Bi-based metal oxide ceramic films used in integrated circuits (ICs). More particularly, the invention-relates to textured Bi-based metal oxide ceramic films with high switchable electrical polarization.
Metal oxide films have been investigated for their use in integrated circuits (ICs). In particular, metal oxide films comprising strontium, bismuth, and tantalum, such as SrBi2Ta2O9 (SBT), have attracted considerable attention because of their excellent ferroelectric properties. The ferroelectric properties of SBT make them a promising material for memory capacitors in nonvolatile ferroelectric random access memory ICs. Various techniques, such as sol-gel, chemical vapor deposition (CVD), sputtering, pulsed laser deposition (PLD), and evaporation, have been developed for depositing such films on a substrate.
Fatigue in the ferroelectric material causes degradation in polarization (2Pr). Degradation in polarization is undesirable as it creates reliability issues in the memory IC. For example, degradation in polarization can result in the signal from the stored charge being too small to be unequivocally defined as a logical xe2x80x9c0xe2x80x9d or xe2x80x9c1.xe2x80x9d To compensate for polarization fatigue, a ferroelectric material with high switchable polarization is needed to increase the reliability of the memory cells.
Other factors also contribute to a need for providing ferroelectric materials with high 2Pr. For example, higher integration densities of devices result in smaller capacitors, necessitating a higher 2Pr value to store the same charge on a smaller capacitor. Also, material degradation due to post-processing can decrease the material""s 2Pr value.
From the above discussion, it is desirable to produce a Bi-based metal oxide with high switchable polarization.
The invention relates to Bi-based metal oxide ceramic layer. In accordance with the invention, the Bi-based metal oxide ceramic layer comprises a crystallographic texture with the correct orientation to result in an increase in the switchable electrical polarization.
In one embodiment, the Bi-based metal oxide ceramic is expressed by YaBibX2Oc, where Y comprises a 2-valent cation and X comprises a 5-valent cation. In one embodiment, Y is equal to one or more elements selected from Sr, Ba, Pb, and Ca. X, in one embodiment, is equal to one or more elements selected from Ta and Nb. Various techniques, such as sol-gel, chemical vapor deposition (CVD), sputtering, pulsed laser deposition (PLD), and evaporation, can be used to form the Bi-based metal oxide. In one embodiment, the Bi-based metal oxide is deposited amorphously by CVD. The amorphous CVD material is post-deposition processed to transform it into a material with the desired electrical properties.
The amorphous layer is processed to produce a ferroelectric layer in accordance with the invention. The amorphous layer comprises materials that can be transformed into a ferroelectric layer. In one embodiment, the amorphous layer comprises a Bi-based oxide ceramic. The Bi-based oxide ceramic comprises, for example, strontium bismuth tantalate (SBT) or a material derived from SBT (SBT derivative). The amorphous layer is annealed under appropriate conditions transforming it into a ferroelectric layer.
Appropriate control of the composition of the Bi-based oxide can result in a highly textured material. In one embodiment, the composition of the Bi-based oxide is controlled to result in a crystallographic texture of the material having an orientation that produces an increase in the average of the components in the polarization direction perpendicular to a conductive layer.
In one embodiment of the invention, the composition of Bi-based metal oxide comprises a Y/2X ratio of about 0.5-0.9, preferably about 0.6-0.8, and more preferably 0.7-0.8. The ratio of Bi/2X, in one embodiment, is about 2.0-2.6, preferably about 2.1-2.5, and more preferably 2.1-2.3.
In accordance with the invention, the Bi-based oxide ceramic film is deposited substantially in amorphous forms, without substantially any fluorite phase present. Depositing an amorphous Bi-based oxide ceramic film without substantially any fluorite phase contradicts conventional wisdom, which suggests that the fluorite phase is crucial as an intermediate phase from which the deposited film is transformed into a ferroelectric. See for example Isobe et al., Integrated Ferroelectric, 1997,Vol. 14, pp. 95-103. There are several reasons why the fluorite phase is believed to important. It is believed that the fluorite phase serves to transport the as-deposited film up to the reaction temperatures required for the perovskite formation without phase separation, e.g., to prevent separation of the oxides during annealing. Additionally, the pre-arrangement of the atoms in a fluorite phase may facilitate a fast film transformation to the Aurivillius phase.
We have discovered that depositing an amorphous Bi-based oxide ceramic film without substantially any fluorite phase is advantageous. For example the amorphous film requires a lower thermal budget than conventional as-deposited films to transform it into the ferroelectric phase. Furthermore, ferroelectrics formed from amorphous films have better electrical characteristics than those formed from conventional techniques.
It is believed that the reason a lower thermal budget is needed to transform the amorphous precursor film into the ferroelectric phase is because amorphous films are more compositionally homogeneous than the fluorite-containing as-deposited films. Heterogeneous films such as the fluorite containing films require more time and higher temperature anneals because some atoms in these film have to move over longer distances. Other detrimental phases might be formed during the longer high temperature anneal in parts of the films with the wrong composition.
In contrast to the conventional as-deposited films, the substantially amorphous as-deposited film is relatively smooth with smaller or no features. The relatively smooth surface indicates that there is reduced Bi segregation during deposition, which produces a film that is more homogeneous in composition than conventional as-deposited films with fluorite phase. The surface morphology of XRD amorphous films is smooth with no or almost no features, indicating that the film is homogeneous in composition.
The amorphous nature of the Bi-based oxide ceramic film is affected by the deposition temperature. The Bi-based oxide ceramic is deposited at a temperature, which produces a substantially amorphous film. In one embodiment, the deposition temperature is lower than 400xc2x0 C., more preferably at about 330-390xc2x0 C., more preferably at about 350-390xc2x0 C., more preferably at about 360-380xc2x0 C., more preferably at about 380xc2x0 C. At temperatures less than 430xc2x0 C., a substantially amorphous film is produced. It has been found, in one embodiment, that an XRD amorphous film is produced when deposited at temperatures of about 380xc2x0 C. or less.
The ferro-anneal transforms the amorphous film into a ferroelectric. A ferro-anneal at a temperature of about 600-830xc2x0 C. for about 1-60 minutes, depending on the temperature, has been found to be sufficient to convert the amorphously deposited film to the ferroelectric Aurivillius phase. The higher the temperature of the anneal, the shorter the time required to transform the substantially amorphous film to the Aurivillius ferroelectric phase. For example, an anneal of about 5 minutes at 750xc2x0 C. is sufficient to complete the transformation to the Aurivillius phase. A slightly longer anneal of about 10 minutes is sufficient to complete the transformation to the Aurivillius phase at about 700xc2x0 C.